An airbag is a vehicle safety device that generally includes a flexible envelope (e.g., a nylon fabric) designed to inflate rapidly during an automobile collision. The airbag's purpose is to cushion occupants during a crash and provide protection to their bodies when they strike interior objects such as the steering wheel, window and/or structural components of the vehicle. Modern vehicles may contain multiple airbags in various side, seat, and/or frontal locations of the passenger and driver seating positions, and sensors may deploy one or more airbags in an impact zone at variable rates based on the type and the severity of impact.
Many airbags are inflated by the ignition of a gas generating propellant via a pyrotechnic device, which rapidly inflates a flexible envelope. The pyrotechnic device usually includes an electrical initiator wrapped in a combustible material and can activate quickly (e.g., less than 2 milliseconds) with a current pulse of about 1 to 3 amperes. When the initiator becomes hot enough, it ignites the combustible material (e.g., a solid propellant). The burning propellant generates inert gas which rapidly inflates the airbag (e.g., the typical rate of inflation in current technology is about 10 to 40 milliseconds). For successful activation, the process requires the pyrotechnic device to generate a high velocity gas that rapidly fills the cushion. The inflation gas will be at high temperature, but relatively low pressure.
A typical combustion gas generant process can include Basic Copper Nitrate (Cu2(NO3)(OH)3) (hereinafter “BCN”) and Guanidine Nitrate C(NH2)3NO3 (hereinafter “GN”) and various mineral based oxides. The combustion process generates three typical gas byproducts, water (H2O), carbon dioxide (CO2) and nitrogen (N2). Nitrogen, which generally acts like an inert gas, is used to inflate the air cushion. In addition, the process generates a range of minerals, these minerals can vary. For example, the most significant minerals generated by the process can be Copper bearing minerals. However, the minerals can also include Aluminum (Al), Chloride (Cl), Cyanate compounds, Iron (Fe), Nitrite compounds, Phosphorus (P), Potassium (K), and Titanium (Ti) bearing minerals. These substances occur in a range of particle sizes from microscopic to 1.5 millimeters (mm), and can also reach generant gas temperatures of 700-900° C. just after ignition. Clearly, these extremely hot particulates, if permitted to enter the air cushion, can cause a catastrophic failure. To prevent this, the current technology introduces a filter that also operated as a diffuser at a point prior to where the generant gas jet exits into the air cushion of the fabric bag. The filter is positioned at this point to capture the particulates generated by the combustion reaction and also absorb some of the heat generated by the combustion, thereby lowering the exit gas temperature and the ultimate temperature reached by the airbag. This filter is usually located inside the air cushion inflator device and may take different shapes according to the air cushion size, shape and location in the vehicle. The inflator device can be high temperature pyrotechnic, pyrotechnic hybrid or simply compressed gas activated. The filter is typically a mechanism made of metal with convoluted passages to permit the inflation gas to exit the inflator while collecting particulates within the passages. The gas and particulates transfer heat via conduction and convection to the filter as the gas exits to inflate the cushion.
Current airbag filter technology mainly consists of stainless wire of various diameters knitted into a rope configuration and then compressed to form a cylinder or some other shape. Filters can also be of a pressed convoluted metal foil design. Compression of the material creates an interference condition between the wires and forms a relatively solid structure with a fairly open tolerance of +/− about 6% on the weight. Due to the method of manufacture, the actual structure of the wire filter is somewhat non-uniform and results in irregular distortion of the wire and consequential creation of internal voids and a general lack of homogeneous wire distribution. While the filter demonstrates the ability to retain some of the particulates generated by the combustion process, the retention occurs in a somewhat irregular and unpredictable manner. The density of the filter is also a significant factor in absorbing heat. In particular, the inflation gas heat transfer process is heavily dependent upon the level of surface contact on the “gas wetted” surfaces. At relatively high densities, the wire can start to impinge on itself, reducing the gas wetted surface available to conduct and/or absorb heat and collect particulates.
Ultimately, the function of the pyrotechnic activation device is to generate a given volume of gas to fill an air bag cushion mechanism in the minimum elapsed time. Rapid generation of the gas occurs via a chemical combustion process that creates high temperature and high velocity, the latter of which ensures the rapid inflation/activation of the air cushion. The inflation gas will be at high temperature, but relatively low pressure. A filter/heat diffuser is employed to capture products of combustion and limit the amount of heat entering the air bag, thereby preventing catastrophic failure due to puncture of the cushion envelope.
The filter typically used in air bags today has a relatively high density and resultant pressure drop. Because of this, an excessive amount of energy is required to force a sufficient volume of gas at very high velocity through the filter in order to inflate the air bag in a sufficient amount of time. This energy in turn translates to a large amount of heat and can cause certain air bag inflator components to reach very high temperatures (e.g., 700-900° C.). Should any of these heated components come into contact with someone or something, they could potentially induce third degree burns or start a fire. Because of these extreme temperatures, the air bag itself must be designed with multiple layers of expensive, highly refractory polymeric materials to act as a heat shield and prevent burns and fires. The relatively irregular structure of the current filter suggests the pressure drop is highly variable from filter to filter creating performance variations.
Current test technology can only produce a pass or fail using a complete functional airbag test which is time consuming and does not easily quantify the relative performance of various sample materials/components. Research suggests the impinging shock wave has a profound effect on performance of heat shield materials and filter/heat diffuser mechanisms. The filter testing apparatus can also permit detailed evaluation of sewn structural integrity of air cushion seams and the development of economic textile alternatives to current seam protection devices.
Although useful, the gravity “hot rod” testing introducing a given mass at a known temperature only provides a very basic thermal resistivity measure and does not replicate the shock wave and high velocity impact experienced when the material is subject to igniter/generant gas jets. Possibly the most detrimental and destructive force impacting the performance of the air bag heat shield is the effect that an impinging under-expanded jet bow shock wave can have on the coated surface and mechanical structure of the heat shield substrate fabric. Couple this effect with the gas having extreme velocity and temperature in excess of 700° C., and the challenge of designing a next generation, robust, efficient heat shield is apparent.
As such a need exists for a filtration device that can be used within an airbag system that provides a more consistent pressure drop from filter to filter, more consistently removes particulates from the high velocity generated gas, and better removes heat from the high velocity generated gas before entry into the fabric air bag of the air cushion of the airbag system.